The present invention relates to impedance matching for monolithic microwave integrated circuits (MMICs).
A difficulty in realizing the potential of MMIC technology is impedance matching. The ubiquity of significant parasitic reactances in MMIC design has no parallel in conventional IC technology, since at microwave frequencies unwanted inductances and capacitances are very easily created by very small changes in the physical size and/or spacing of components.
While it is of course possible to use conventional matching stages off-chip to compensate for impedance mismatches produced by such parameter variation, such matching networks are necessarily bulky and would destroy much of the advantage obtained by using MMICs in the first place. Alternatively, if no matching network is used, the performance of circuits including an MMIC, and particularly that of circuits including multiple MMICs, would be degraded. Similarly, a substantial percentage of MMICs manufactured would have to be rejected because their impedance characteristics were too far away from the design values.
It is thus an object of the invention to provide an impedance matching stage which can easily be incorporated within an MMIC.
If a matching network is used, the time required to tune the matching network must not consume too large a percentage of the manufacturing time. Ideally, it should be possible to program the impedance characteristics of the MMIC after all major manufacturing steps have been completed. A very convenient method of doing so would be to have a voltage-programmable impedance characteristics, so that a chip which had completed the major stages of manufacture could be connected to a chip tester which would, while testing the chip for faults, also determine the correct programming voltage to produce the desired output impedance characteristics.
Thus, it is a further object of the present invention to provide an MMIC which has voltage-programmable impedance characteristics.
A related difficulty arises when it is desired to operate an MMIC over a very wide band of frequencies, not necessarily simultaneously. For example, a communications receiver which is to down convert different bands must, aside from the hardware required for band-switching, have some means to provide at least tolerable impedance matches at each of the operating bands.
Thus, it is a further object of the present invention to provide means for dynamically reprogramming the impedance characteristics of MMICs which are operated over a very wide frequency range.
A particulary acute problem in the development of MMICs, to which no satisfactory solution has yet been found, is temperature compensation. At present, this is conventionally accomplished by using one or more thermistors to control a p-i-n diode network off-chip, providing variable attenuation. However, not only is this solution bulky, it also permits only one attenuation (temperature) function for each off-chip thermistor module. Since temperature compensation should optimally be different for different MMIC circuits, it would be highly desirable to have some method of on-chip temperature compensation which could be tailored to the temperature curves of the particular MMIC circuit involved.
It is thus a further object of the present invention to provide on-chip temperature compensating elements for MMICS.
It is a further object of the present invention to provide on-chip temperature compensating elements for MMICs which can provide various selected compensating functions, in response to a single externally generated voltage signal indicative of temperature.
In assembling microwave modules, the intrinsically variable impedance characteristics of microwave chips, resulting from the normal parameter variations which occur in manufacturing, create difficulties in properly matching interfaced components of the module together. Either additional matching elements must be used, which is bulky and time consuming as discussed above, or other steps must be taken to avoid excessive mismatch between adjacent elements. At present, the most common method of assembling such microwave modules (e.g. where a plurality of MMIC chips are to be united in a thin film structure on an aluminum substrate) is to first characterized chips produced in the production run. Impedance matches among the chips as tested are then made by sorting, to assemble as many acceptably matched modules as possible. Some of the remaining chips then have their parameters altered, and a further sorting process is then made to attempt to find new matches. This process is necessarily time consuming and expensive, and also necessitates use of batch production methods.
Thus, it is a further object of the present invention to provide MMICs, such that the MMICs can be assembled into microwave modules without any step of sorting the MMICs according to their impedance characteristics being required.
Implementation of microwave systems frequently requires circulators, to interface different functional modules while maintaining acceptable isolation. However, circulators are bulky and extremely expensive, and it would be desirable to minimize their use. This could be accomplished if the overall VSWR of the respective modules could be improved.
Thus, it is a further object of the present invention to provide MMICs which, when assembled into modules, will minimize the VSWR for the whole module.
In particular, one MMIC in which very wide-band frequency response is desirable, and to which varactor trimming for output impedance-matching is particularly applicable, is a monolithic microwave wide-band VCO. If an integrated varactor is used in such a wide-band monolithic VCO, an additional varactor for output impedance-match trimming can easily be formed simultaneously by the same process steps. However, it has not hitherto been possible to form a monolithic varactor which could easily be integrated in an MMIC and which had adequately wide-band impedance characteristics.
Conventional varactor diodes, particularly those with large tuning ratios (hyperabrupt diodes) require highly conductive substrate material and relatively thick epitaxial layers (greater than one micron). These material requirements are not compatible with the requirements of GaAs FET-monolithic microwave integrated circuits (MMICS) which require a thin (less than one-half micron) uniformly doped active layer on a semi-insulating substrate. To integrate the conventional hyperabrupt diode on a semi-insulating substrate requires a very complicated selective epitaxial deposition, wherein certain areas of the substrate surface receive one epitaxial layer, and other areas receive a different epitaxial layer. The materials required to implement a varactor in an MMIC should be the same as or similar to those for an FET, so that varactors can easily be integrated in, e.g., monolithic microwave voltage controlled oscillators.
Thus, it is an object of the present invention to provide a monolithic microwave integrated circuit incorporating a wide-ratio varactor in a thin uniformally doped active layer above a semi-insulating substrate.
R. VanTuyl, "A Monolithic GaAs FET RF Signal Generation Chip", ISSCC-80 Digest 118 (which is hereby incorporated by reference) discloses a gallium arsenide varactor diode in an MMIC which is integrated in a thin epitaxial layer on a semi-insulating substrate. The VanTuyl device does not, however, provide very wide capacitance tuning characteristics. A wide capacitance range (of a decade or more) is essential for many microwave applications. In addition, the VanTuyl device is designed for operation only at lower microwave frequencies (of at most 4 GHz).
The frequency tuning range of a varactor-based VCO is much narrower than the capacitance range of the tuning varactor, due to the inherent and parasitic reactance characteristics of FETs and other components of the VCO. In particular, an extremely wide-range varactor (having a capacitance ratio of a decade or more) is needed if the frequency range of the VCO is to remotely approach one octave.
Thus, it is an object of the present invention to provide a VCO having a tuning range of 1.5 to 1 or larger at microwave frequencies. It is a further object of the present invention to provide a VCO having a tuning range of 1.3 to 1 or better at microwave frequencies above 5 GHz.
It is a further object of the present invention to provide a microwave VCO having a tuning range of an octave or more.
It is a further object of the present invention to provide a monolithic microwave VCO having a tuning range of 1.5 to 1 or larger.
A major difficulty which arises in microwave VCOs having such a large frequency range is maintaining the correct impedance match to achieve the maximum obtainable bandwidth. Mismatch can easily become such as to gravely impair performance.
However, in a monolithic microwave integrated circuit even trimming (to achieve impedance match at one particular frequency) is difficult, and optimal matching over a wide range of frequencies is presently impossible.
Thus, it is a further object of the present invention to provide means for maintaining impedance matching of a monolithic microwave wide-band VCO over a very large frequency range.